The invention relates to a two-stroke internal combustion engine with crankcase scavenging, with the exhaust passage controlled by the piston and at least two first transfer passages and at least one second transfer passage, each with a transfer window into the cylinder chamber and a window on the crankcase side for communication between the cylinder chamber and the crankcase, the transfer windows of the first transfer passages being controlled by the upper edge of the piston, and the crankcase-side window of the second transfer passage being disposed in a region of the cylinder wall that is swept by the piston skirt of the piston, wherein for control of the second transfer passage the piston skirt of the piston is provided with a control opening in the area of the crankcase-side window of the second transfer passage.
The U.S. Pat. No. 3,881,454 describes a two-stroke engine construction with a pair of main scavenging passages and an auxiliary scavenging passage, in which a fuel injector is located. The piston is reciprocatable to open and close the exhaust, main and auxiliary scavenging passages, which communicate at one end with the crankcase at least as long as the auxiliary scavenging window is uncovered by the piston. As the fuel injector is arranged near the entry of the auxiliary scavenging passage into the combustion chamber the temperature of the nozzle becomes very high resulting in carbon deposits and higher wear.
In AT 399 913 B a two-stroke engine is described, where at least two of the transfer or scavenging passages open earlier than the others to obtain the desired scavenging results, i.e., at a time when the pressure in the cylinder still exceeds that in the crankcase, so that exhaust gas will flow into these passages and push back the fresh charge inside without entering the crankcase itself. The timing of these transfer passages is effected by the upper edge of the piston acting as a control edge. Fuel supply is effected via a carburetor into a feed passage connected with the crankcase.
In AT 397 695 B an internal combustion engine is described, where a fuel injector is positioned in the transfer passage, the injected fuel stream being directed towards the side of the piston top facing the cylinder chamber. The axis of the injected fuel stream forms an angle with the piston axis that is smaller than 90xc2x0, so that the fuel will mostly hit the half of the flat and rotation-symmetrical piston top opposite of the exhaust window. The drawback of this configuration is that strong scavenging losses are incurred in the lower speed range. Moreover, the time available for an injection cycle is limited by the opening period of the scavenging windows. For selection of the injector this may lead to problems with the dynamic range, i.e., the ratio of idle quantity to full-load quantity. Because of the shortness of the injection period the fuel is injected onto the piston edge and piston rings, which will have negative effects on HC emissions and cause wetting of the cylinder wall and wash off the lubrication film.
DE 196 27 040 A1 presents a two-stroke spark-ignition engine with fuel injection, where an injector is positioned in the cylinder wall next to the cylinder chamber. This arrangement and the lack of air circulation around the injector will lead to high temperatures and an increase in carbon deposits at the injector nozzle. Other undesirable results, which are caused by the direction of the injected fuel stream, will be the wetting of the cylinder wall and washing-off of the lubricating film.
Another two-stroke internal combustion engine is described in U.S. Pat. No. 5,443,045, where an injector passes through the cylinder wall into the cylinder. The injector faces the exhaust window at a downward angle, which will result in strong scavenging losses in the lower speed range. As the injector is positioned in that area of the cylinder wall that is swept during the compression stroke high temperatures and increased build-up of carbon deposits at the injector nozzle will occur due to the lack of air circulation.
It is an object of the invention to overcome these disadvantages and to propose a two-stroke internal combustion engine of the above type, where fuel losses and hydrocarbon emissions may be minimized in a simple manner for both high and low speeds and loads. Another object of the invention is to increase the service life of the injector.
According to the invention this is achieved by providing that said cylinder wall comprises at least one recess and that at least one first injector opens into said recess being arranged in close vicinity to said transfer window of said second transfer passage, wherein a narrow passage is arranged between said recess and said second transfer passage. Disposing the first injector in the recess which communicates with the second transfer passage will considerably reduce carbon deposits as the air circulation prevailing in this region will afford cooling, and will enhance carburetion. Due to the narrow passage air flowing from the second transfer passage to the recess will be accelerated and therefore improve the cooling of the orifice of the first injector.
It is provided in a preferred variant that the narrow passage is shaped by at least one groove in the cylinder wall. According to an alternative embodiment the recess may also be shaped by a duct branching off from the second transfer passage and leading to the recess. If the recess is disposed above the transfer window of the second transfer passage, i.e., on the side of a combustion chamber top, the groove may be arranged in the direction of the cylinder axis.
According to another embodiment of the invention the narrow passage is shaped by a separating edge of a cross rib, said separating edge dividing the recess from said transfer window of said second transfer passage. The distance between the separating edge and the cylinder axis is larger than the distance between the cylinder wail and the cylinder axis.
According to a further embodiment of the invention a reduction of carbon deposits can be achieved by means of a separating edge being located in at least one transfer passage between an upper and a lower edge of the second transfer window. The separating edge divides the second transfer window into an upper and lower region, wherein the separating edge is at a larger distance from the cylinder axis than the upper or lower edge. If the edge of the piston passes the separating edge, the lower region of the second transfer window will be closed. Between the separating edge and the piston there is a small gap through which the intake flow passes into the upper region, flowing around and cooling the nozzle. The separating edge is formed by a cross rib, being arranged at right angle to the cylinder axis.
The axes of the first injectors may be positioned within a wide angular range of xcex1=20-140xc2x0 relative to the cylinder axis or a parallel thereof. Moreover, the directions of the axis of the fuel stream coming from the first injector, i.e., the jet axis, and the injector axis of the first injector may differ. In this way the injection characteristics of the first injector may be adjusted to the respective requirements.
It is provided in a preferred variant of the invention that at least two second transfer passages be disposed on the side of the exhaust passage and, preferably, that the first injector opening into at least one recess or one second transfer passage faces the half of the cylinder chamber opposite of the exhaust passage. In this manner scavenging losses may be kept exceedingly small.
In another variant of the invention the proposal is put forward that at least one second transfer passage be disposed on the side of the cylinder opposite of the exhaust passage. The second transfer passage may be positioned in a symmetry plane containing the axis of the exhaust passage and the cylinder axis.
To keep scavenging losses as small as possible it could also be provided that at least two second transfer passages be positioned outside of a symmetry plane containing the exhaust passage axis and the cylinder axis. In this context the jet axis of the first injectorxe2x80x94seen in the direction towards the cylinder axisxe2x80x94should preferably be directed onto a transfer window of a transfer passage that is preferably diametrically opposed.
In order to prevent the cylinder wall from being wetted by the fuel it may be provided that the jet axis of the first injectorxe2x80x94seen in the direction towards the cylinder axisxe2x80x94be directed onto the exhaust window of the exhaust passage.
To enable the first and second transfer passages to be timed independently of each other, and to permit optimum fuel injection into the cylinder chamber it may be provided that the upper edge of the transfer window of the second transfer passage be at a smaller distance from the top of the combustion chamber than the upper edge of the transfer window of the first transfer passage. This configuration is especially suitable for use with rotation-symmetrical, flat piston tops.
In the instance of an internal combustion engine with several first injectors per cylinder it may be provided by the invention that the injection axes of the injectors disposed in second transfer passages have different angles relative to each other and/or to the cylinder axis or a parallel of the cylinder axis. In this way first injectors with different injection characteristics may be used, permitting carburetion to be adjusted to any operating state of the engine. One first injector could be configured as full-load injector and another one as part-load injector, for example.
For optimum adjustment of the injection process to the respective requirements it may be provided in further development of the invention that the first injector be configured as a multi-jet unit, with at least two fuel jets differing as regards fuel amounts and/or injection times, preferably.
Designing the internal combustion engine as proposed by the invention will ensure that the transfer windows of the first and second transfer passages and the crankcase-side window of the at least one second transfer passage and the corresponding control opening are disposed such that the first and second transfer passages will be timed in different stroke positions of the piston. Obviously, the transfer passages could also be arranged so that the first and second transfer passages are controlled roughly simultaneously though by means of different control edges.
According to a further embodiment of the invention there is further provided at least one second injector opening into a crankcase chamber. The second injector may also open into an inlet duct, wherein the second injector can be arranged downstream of a reed-valve or upstream of the reed-valve and downstream of a throttle valve. At low and middle load as well as at low and middle speed fuel is injected via the first injector. At high or full load and at high or full speed fuel is injected at least by means of the second injector.